1. Field of the Invention
The present invention relates to an organic electroluminescence element (which may be referred to as an organic EL element hereinafter). More specifically, the present invention relates to an organic EL element using a triplet exciton of an organic luminescence material (host material).
2. Description of the Related Art
Hitherto, organic EL elements wherein an organic luminescence layer is arranged between electrodes have been eagerly researched and developed for the following reasons and the like.
(1) Since these elements are completely solid, they are easy to handle and produce.
(2) Since they can emit light by themselves, no light emitting members are necessary.
(3) Since they can be clearly watched, they are suitable for display.
(4) They permit full color display easily.
The luminescence mechanism of such organic EL elements generally makes use of a luminescence phenomenon, which is energy conversion phenomenon caused when a fluorescent molecule in a singlet excited state (which may be referred to a S1 state) in an organic luminescence medium is transited to a ground state radially.
A fluorescent molecule in a triplet excited state (which may be referred to a T1 state) in an organic luminescence medium can be supposed. However, radiative transition to a ground state is forbidden; therefore, such a molecule is gradually transited from the triplet excited state to some other state by non-radiative transition. As a result, no fluorescence is emitted but thermal energy is radiated.
Here, singlet and triplet mean multiplicity of energy decided by combination of total spin angular momentum and total orbital angular momentum of a fluorescent molecule. Specifically, a singlet excited state is defined as an energy state in the case that a single electron is transited from a ground state, where no unpaired electrons are present, to a higher energy level without changing the spin state of the electron. A triplet excited state is defined as an energy state in the case that a single electron is transited to a higher energy level while the spin state of the electron is made reverse.
Needless to say, luminescence in a triplet excited state defined as above can be observed if the luminescence is caused at a very low temperature, for example, at a liquefaction temperature of liquid nitrogen (−196° C.). However, this temperature is not a practical temperature, and the amount of the luminescence is only a little.
By the way, the total efficiency of luminescence from any conventional organic EL element is related to recombination efficiency (φrec) of injected charged carries (electrons and holes), and the probability (φrad) that generated excitons cause radiative transition. Therefore, the total efficiency (φel) of luminescence from the organic EL element can be represented by the following equation:φel=φrec×0.25φrad 
The coefficient (0.25) of φrad in the equation is decided from the matter that the probability that singlet excitons are generated is regarded as ¼. Therefore, even if recombination and radiative attenuation of excitons are caused with a probability coefficient of 1, the theoretical upper limit of luminescence efficiency of the organic EL element is 25%.
As described above, in any conventional organic EL element, triplet excitons cannot be substantially used and only singlet excitons cause radiative transition. Thus, a problem that the upper limit of the luminescence efficiency is low arises.
Thus, literature 1 “Jpn. J. Appl. Phys., 38 (1999) L1502” discloses that even at room temperature, triplet excitons (triplet excited state) of an organic luminescence material (host material) are used to transfer energy from the triplet excitons to a phosphorescent dopant, so as to generate a fluorescent luminescence phenomenon. More specifically, the literature 1 reports that a fluorescent luminescence phenomenon is caused in an organic EL element comprising an organic luminescence layer composed of 4,4-N,N-dicarbazolylbiphenyl represented by the following formula (6) and an Ir complex, which is a phosphorescent dopant.

However, the half-life of the organic EL element described in the literature 1 is below 150 hours, and the usefulness of the organic EL element is insufficient.
Thus, the inventor made eager investigations. As a result, the following has been found: the glass-transition temperature of 4,4-N,N-dicarbazolylbiphenyl is as low as less than 110° C.; therefore, if the biphenyl is combined with an Ir complex, crystallization is easily caused in the organic luminescence layer comprising the combination to make the life of an organic EL element short.
Incidentally, in the present situation, a demand that the heat-resistance of organic EL elements for cars should be made higher has been increasing in light of environment inside cars in summer.
Thus, an object of the present invention is to provide an organic EL element which makes it possible to use triplet excitons of an organic luminescence material (host material) even at room temperature to emit fluorescence (including phosphorescence); has a practical life span; and has a superior heat-resistance.